Reference is made to FIG. 1 which illustrates a circuit diagram for a conventional Class-D amplifier 10 operable to convert an input signal 12 (for example, an audio signal) into high-frequency pulses 14. A typical Class-D amplifier utilizes a pulse width modulator 18 to generate high-frequency pulses 14 that vary in width as a function of the amplitude of the input signal 12. The pulse width modulator 18 may typically comprise a comparator 19 having a first (positive) input receiving the input signal 12 and a second (negative) input receiving a sawtooth (or triangular) waveform reference 21. The varying-width pulses output from the pulse width modulator 18 are processed by a drive logic circuit 20 to produce opposite phase pulsed control signals 22 for application to the control terminals of transistor switches 16 arranged in a half-bridge configuration. An output of the transistor bridge is coupled through a low pass filter 24 (with a DC blocking capacitor) to a load 26 (in this case illustrated as a speaker for when the input signal is an audio signal). The lowpass filter 24 converts the pulses back into an amplified version of the input signal for application to the load. In an implementation, the filter 24 is formed of a conventional inductive/capacitive circuit, although more complex filtering circuitry could be used if desired.
Although a pulse width modulator 18 is illustrated as the circuit to convert the input signal into high-frequency pulses, it is known in the art to utilize other pulse modulation circuitry to process the audio input signal. For example, a pulse density modulator could be used.
With reference to FIG. 2, the conventional Class-D amplifier 10 may further include an integrator circuit 30. The integrator circuit 30 comprises an operational amplifier 32 having a first (negative) input coupled to receive the input signal 12 through an input resistor Rin. The second (positive) input of the operational amplifier 32 receives a fixed reference voltage Vref. The output of the operational amplifier is coupled to the input of the pulse width modulator 18, and is further coupled in a feedback circuit to the negative input of the operational amplifier 32 through a feedback capacitor Cfb. The fixed voltage Vref supplied to the positive input of the operational amplifier 32 is typically set equal to one-half of the supply voltage Vdd for the operational amplifier 32. The supply voltage Vdd is typically separate from, and at a different voltage level than, the supply voltage Vcc used by the transistor bridge.
In operation, the square-wave output of the switching power transistors at the transistor bridge is summed with the audio input at the negative input of the operational amplifier 32 to provide a negative feedback. This negative feedback is taken before the lowpass filter 24 (rather than after) so as to avoid the need in the feedback loop for a complicated compensation network to handle the phase shift introduced by the lowpass filter. A feedback resistor Rfb is accordingly coupled between the output of the transistor bridge and the negative input of the operational amplifier 32.
The square-wave output of the transistor bridge is synchronous with the audio input, but it is important to remove the carrier of the audio input signal. The integrator circuit 30 functions to sum the square-wave output and audio input signal. The integrator circuit 30 feeds the resultant error signal into the positive input of the duty cycle modulator 18. The comparator circuit of the modulator 18 accordingly compares the triangle waveform reference to the error signal and produces the modulated output as a square wave whose duty cycle is proportional to the amplitude of the audio input signal.
In order to properly drive the transistor bridge circuit, the drive logic 20 converts the modulated output to drive signals for driving the upper and lower power switches of the transistor bridge in an antiphase relationship. The drive logic 20 will accordingly drive one switch of the bridge into saturation while the other switch of the bridge is cut off (and vice versa). As those skilled in the art know, the combination of switching and conduction losses for the transistor bridge defines the upper bound of the amplifier's efficiency. The square wave of the modulated output causes the bridge switches to change state as fast as possible. Fast switching is desired because it limits the time that the bridge switches spend in the linear operating region, thereby increasing efficiency and reducing heat generation.
The lowpass filter 24 functions to filter out the high-frequency square wave that the power switches of the transistor bridge generate. This leaves only an amplified version of the input audio signal to drive the load.
Those skilled in the art recognize that Class-D amplifiers produce a noticeable “pop” at the speaker load when power to the amplifier is first turned on. It is important to maintain fidelity in the output signal with respect to the input audio signal. The presence of any artifact, such as the “pop” at power on, in the output signal is unacceptable.
While a number of solutions to the start-up artifact problem are known in the art, many of these solutions are expensive, unduly complex or may introduce other problems (including artifacts). There is a need in the art to provide an inexpensive and efficient solution to the start-up artifact problem associated with conventional Class-D amplifiers.